1. Field of the Invention
The present invention relates to a dielectric resonator device including a dielectric plate having a plurality of resonance regions, and to a dielectric filter, a dielectric duplexer, and a communication apparatus including the dielectric resonator device.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 11-234008 discloses a known flat-circuit dielectric resonator device. The dielectric resonator device includes a dielectric plate. An electrode is provided on each of both principal surfaces of the dielectric plate and mutually-opposing openings are formed in the electrodes. An electrode opening functioning as a resonator of an input/output unit is formed in a slot shape extending in the direction of each end surface of mutually-opposing shorter-sides of the dielectric plate. Also, the resonators are linearly aligned in the direction parallel to a magnetic field direction when the resonators couple in a magnetic field.
FIGS. 13A and 13B show the configuration of the dielectric resonator device. Herein, reference numeral 2 denotes an electrode which is formed on the upper surface of the dielectric plate and which includes electrode openings 4a, 4b, and 4c. This dielectric resonator device includes a three-stage resonance region. The first and third stages on both sides serve as resonators using the electrode openings 4a and 4c, one edge of the openings 4a and 4c being open in the shorter-sides of the dielectric plate. The second stage serves as a resonator using the electrode opening 4b, both edges thereof being closed so as to form a rectangular shape.
A used resonance frequency is defined so that the following expressions are satisfied: L=about (2n−1)/4 wavelength (n is an integer which is one or more, and L is the length in the longer-side direction of each resonator) when one edge is opened, and L=about n/2 wavelength (n is an integer which is one or mode) when the both edges are closed so as to form a rectangular shape.
Further, input/output coupling probes 11 and 12 connected to an input/output terminal are provided in a direction perpendicular to the magnetic field of the resonator defined by the electrode openings, at the open-end side of the electrode openings 4a and 4c. 
The above-described dielectric resonator device can be used as a very compact and lightweight filter. However, if a filter having a different resonance frequency fo is designed in a system of the same frequency band by using the configuration shown in FIG. 13A, the length of the longer-side of the dielectric plate must be changed. For example, as shown in FIG. 13B, when fo is decreased, the length L in the longer-side direction of the electrode openings 4a, 4b, and 4c increases to L′. Accordingly, the length of the dielectric plate increases and the size of the filter also increases. As a result, the position of the input/output terminal must be changed and standardization of a mounting pattern on a circuit board cannot be realized.
The standardization can be realized if the filter is designed to be large considering the change in the size of the dielectric plate. In that case, however, needs for miniaturization cannot be satisfied.